The ATV (Advanced Television) standards proposed for the next-generation digital broadcasting system of the U.S. are to adopt the MPEG (Moving Picture Image Coding Experts Group) method, which is an international standard for image compression, as a compression method for use in transmitting image data. DVB (Digital Video Broadcasting), the next-generation broadcasting of Europe, is also going to adopt the MPEG method.
For data coding, the MPEG system uses motion-compensated prediction, DCT (discrete cosine transform) and variable-length coding. As shown in the frame structure of FIG. 21, pictures are classified into three types, i.e., I pictures (intra-coded) pictures, P (predictive coded) pictures and B (bi-directionally predictive coded) pictures, according to the time-base prediction node. The I picture is an intraframe coded picture, the P picture is an interframe predictive coded picture which is predicted from a frame ahead in time, and the B picture is all interframe predictive picture predicted from frames preceding and following the current frame.
FIG. 22 shows the hierarchical structure of image data of the MPEG method. The structure comprises the following layers as arranged from below upward.
Block Layer
A block comprises 8.times.8 pixels which are adjacent in luminance or color difference, and DCT is performed in the unit of block.
Macroblock Layer
A macroblock comprises six blocks, i.e., adjacent four luminance blocks and two color difference blocks Cb, Cr coinciding in position on the screen.
Slice Layer
A slice comprises a plurality of macroblocks arranged successively in the order of image scanning.
Picture Layer
A picture comprises a plurality of slices to form an image. Pictures are classified into I pictures, P pictures and B pictures according to the coding method as previously stated.
GOP Layer
GOP (group of pictures) comprises one or a plurality of I pictures, and zero or a plurality of non-I pictures.
Video Sequence Layer
A video sequence comprises one or a plurality of GOPs which are identical in image size, image rate or the like.
On the other hand, studies are conducted by the HD (High Definition) Digital VCR Conference on the standardization of digital VTRs for recording video signals as digital data and reproducing video signals. In this connection, it is thought feasible to record on magnetic tapes data of ATV standards (ATV signals) subjected to image compression by the MPEG method, or DVB signals.
FIG. 23 shows the standard signal recording format used for digital VTRs. As illustrated, the track formed on the signal bearing surface of a magnetic tape 71 comprises an ITI area containing insert data and track data, aural data area, image data area and subcode data area. In PAL mode, 360 tracks are recorded on the magnetic tape per second, and the image of one frame is recorded with use of 12 tracks on the average.
Data is recorded in units which are termed sync blocks. Recordable in the image data area of one track are 135 sync blocks. As shown in FIG. 24, each sync block comprises a sync data area (2 bytes), ID data area (3 bytes), image data area (77 bytes) and parity area (8 bytes), i.e., 90 bytes in total.
In the case where MPEG data is recorded in the image data area, the MPEG bit stream input is recorded in the form of the data hierarchy shown in FIG. 22 and includes macroblock units. These macro-blocks are not definite in code length and are recorded along with upper layers, so that the sync block described and the macroblock to be recorded are not in a definite relationship. Thus, the position on the screen that is determined by the macroblock is irrelevant to the position on the recording track pattern that is determined by the sync block.
In the case where a magnetic tape having data thus recorded thereon is caused to travel at a speed different from the recording speed for trick playback, for example, for high-speed playback, the head scans the tape over a plurality of tracks, so that the MPEG data obtained is not successive but fragmentary. The MPEG bit stream includes P pictures and B pictures which are interframe predictive coded pictures as previously stated, with the result that the fragmentary data available fails to reconstruct images.
Stated more specifically, trick playback pictures can be reconstructed only from I pictures which are intraframe predictive coded pictures, and for the reconstruction from I pictures, all I picture data distributed over the track needs to be retrieved in the correct order. Accordingly, the method wherein the MPEG bit stream input is recorded as it is infeasible for trick playback.
"A Recording Method of ATV Data on a Consumer Digital VCR," a paper delivered at "International Workshop On HDTV '93" held in October, 1993 discloses a method which is known as a method of trick playback in digital VTRs.
With this method, specific areas are provided within the image data area of each track, and I picture data is recorded in the specific areas as trick playback data aside from usual playback data. This method of recording is feasible because the data rate for the ATV signal is about 19.4 Mbps, whereas the image data area of one track according to the standards of the HD Digital VCR Conference is adapted for recording at a data rate of about 24.9 Mbps, so that a region corresponding to about 5,.5 Mbps remains as a surplus recordable area. The surplus area corresponds to about 32 SBs (sync blocks) relative to the image data area of one track which corresponds to 135 SBs.
Stated more specifically with reference to FIG. 25, provided in the image data area of one track are data areas NPA for usual playback, and data areas TPA for trick playback at three locations on the track, i.e., at front, central and rear portions thereof. Of the 135 SBs for the image data area of one track, 32 SBs in total are assigned to the three trick playback data areas TPA.
In this case, trick playback data is assigned by the method shown in FIG. 26. First, macroblocks of I pictures A, B, C, . . . are extracted from an MPEG bit stream as shown in FIG. 26(a) to obtain a row of data A', B', C', . . . as seen in FIG. 26(b). The macroblocks as coded are different in code length. The data row A', B', C', . . . is then successively assigned to sync blocks SB1, SB2, SB3, . . . as shown in FIG. 26(c). These sync blocks have a definite code length and are therefore not in a definite relationship with the macroblocks; one macroblock will correspond to a plurality of sync blocks.
The same trick playback data is recorded on tracks which are equal in number to the maximum number of speed multiplication, n, to be set for trick playback. For example, when n=5, the same data is recorded in the track play data areas PTA of five successive tracks as shown in FIG. 25. The data is refreshed for every 5-track unit, and this procedure is repeated a number of times (m times) to record all I picture data with 5.times.m tracks.
The whole I picture data recorded by the above method can be obtained by scanning each of m 5-track units once to scan the 5.times.m tracks if the playback speed is up to the 5-fold speed.
With DVB (Digital Video Broadcasting) to be adopted in Europe, the bit rate of broadcasting signal of one program is variably settable at a desired value such as 20 Mbps, 10 Mbps or 4 Mbps. With digital VTRs, on the other hand, signals can be recorded at a bit rate of 25 Mbps. Accordingly, when the DVB signal (usual playback data), which is lower than 19.2 Mbps in bit rate, is recorded as it is; on a magnetic tape by the digital VTR, there occurs a signal absent region with an area corresponding to the reduction of the bit rate, hence a problem in efficient use of the tape.
Further according to the ATV method, a bit stream (image data 1V, aural data 1A and code data 1D) relating only to one program is broadcast over a period of time as shown in FIG. 20A, whereas with the DVB method, a bit stream (1V, 2V, 3V, 1A, . . . ) involving a plurality of programs, e.g., four programs, is broadcast as seen in FIG. 20B. Accordingly, if the DVB signal involving such programs is recorded by the digital VTR on a tape ailing the conventional method of FIG. 25 as it is, trick playback data as to different programs will be randomly present in the plurality of trick playback data areas to be arranged in the path to be traced by one head for trick playback. As a result, when one of the programs is to be reproduced by trick playback, there arises the problem that it is impossible to obtain all the trick playback data (I picture data) as to that program and to perform trick playback.
Further when it becomes no longer necessary to preserve the data as to one of the programs recorded on the tape, it is impossible to erase the data of the unnecessary program only and to record data of another program in the erased areas because the programs are randomly recorded on the tape. Thus, another problem is encountered in the efficient use of the tape.